M.H. Lee

DISCREPANT HARDENING OBSERVED IN COSMIC-RAY ELEMENTAL SPECTRA

The balloon-borne Cosmic Ray Energetics And Mass experiment launched five times from Antarctica has achieved a cumulative flight duration of about 156 days above 99.5% of the atmosphere. The instrument is configured with complementary and redundant particle detectors designed to extend direct measurements of cosmic-ray composition to the highest energies practical with balloon flights. All elements from protons to iron nuclei are separated with excellent charge resolution. Here, we report results from the first two flights of ~70 days, which indicate hardening of the elemental spectra above ~200 GeV/nucleon and a spectral difference between the two most abundant species, protons and helium nuclei. These results challenge the view that cosmic-ray spectra are simple power laws below the so-called knee at ~1015 eV. This discrepant hardening may result from a relatively nearby source, or it could represent spectral concavity caused by interactions of cosmic rays with the accelerating shock. Other possible explanations should also be investigated.

Cosmic-ray Proton and Helium Spectra from the First CREAM Flight

Cosmic-ray proton and helium spectra have been measured with the balloon-borne Cosmic Ray Energetics And Mass experiment flown for 42 days in Antarctica in the 2004–2005 austral summer season. High-energy cosmic-ray data were collected at an average altitude of �38.5 km with an average atmospheric overburden of �3.9 g cm −2 . Individual elements are clearly separated with a charge resolution of �0.15 e (in charge units) and �0.2 e for protons and helium nuclei, respectively. The measured spectra at the top of the atmosphere are represented by power laws with a spectral index of 2.66 ± 0.02 for protons from 2.5 TeV to 250 TeV and –2.58 ± 0.02 for helium nuclei from 630 GeV nucleon −1 to 63 TeV nucleon −1 . They are harder than previous measurements

ENERGY SPECTRA OF COSMIC-RAY NUCLEI AT HIGH ENERGIES

We present new measurements of the energy spectra of cosmic-ray (CR) nuclei from the second flight of the balloon-borne experiment Cosmic-Ray Energetics And Mass (CREAM). The instrument included different particle detectors to provide redundant charge identification and measure the energy of CRs up to several hundred TeV. The measured individual energy spectra of C, O, Ne, Mg, Si, and Fe are presented up to ~1014 eV. The spectral shape looks nearly the same for these primary elements and it can be fitted to an E –2.66 ± 0.04 power law in energy. Moreover, a new measurement of the absolute intensity of nitrogen in the 100-800 GeV/n energy range with smaller errors than previous observations, clearly indicates a hardening of the spectrum at high energy. The relative abundance of N/O at the top of the atmosphere is measured to be 0.080 ± 0.025 (stat.)±0.025 (sys.) at ~800 GeV/n, in good agreement with a recent result from the first CREAM flight.

Measurement of the cosmic-ray low-energy antiproton spectrum with the first BESS-Polar Antarctic flight

Abstract The BESS-Polar spectrometer had its first successful balloon flight over Antarctica in December 2004. During the 8.5-day long-duration flight, almost 0.9 billion events were recorded and 1,520 antiprotons were detected in the energy range 0.1–4.2 GeV. In this Letter, we report the antiproton spectrum obtained, discuss the origin of cosmic-ray antiprotons, and use antiproton data to probe the effect of charge-sign-dependent drift in the solar modulation.

Measurement of the cosmic-ray antiproton spectrum at solar minimum with a long-duration balloon flight over Antarctica

The energy spectrum of cosmic-ray antiprotons (ps) from 0.17 to 3.5 GeV has been measured using 7886 ps detected by BESS-Polar II during a long-duration flight over Antarctica near solar minimum in December 2007 and January 2008. This shows good consistency with secondary p calculations. Cosmologically primary ps have been investigated by comparing measured and calculated p spectra. BESS-Polar II data show no evidence of primary ps from the evaporation of primordial black holes.

Search for Antihelium with the BESS-Polar Spectrometer

In two long-duration balloon flights over Antarctica, the Balloon-borne Experiment with a Superconducting Spectrometer (BESS) collaboration has searched for antihelium in the cosmic radiation with the highest sensitivity reported. BESS-Polar I flew in 2004, observing for 8.5 days. BESS-Polar II flew in 2007-2008, observing for 24.5 days. No antihelium candidate was found in BESS-Polar I data among 8.4×10(6) |Z|=2 nuclei from 1.0 to 20 GV or in BESS-Polar II data among 4.0×10(7) |Z|=2 nuclei from 1.0 to 14 GV. Assuming antihelium to have the same spectral shape as helium, a 95% confidence upper limit to the possible abundance of antihelium relative to helium of 6.9×10(-8)} was determined combining all BESS data, including the two BESS-Polar flights. With no assumed antihelium spectrum and a weighted average of the lowest antihelium efficiencies for each flight, an upper limit of 1.0×10(-7) from 1.6 to 14 GV was determined for the combined BESS-Polar data. Under both antihelium spectral assumptions, these are the lowest limits obtained to date.

Study of the internal background of CsI(Tl) crystal detectors for dark matter search

A search for particle cold dark matter with CsIðTcÞ crystal is being prepared at the Cheong-Pyeong underground laboratory in Korea. The background spectra ofCsI ðTcÞ crystal detectors in a prototype shield were obtained. The lowest background count rate ofthe test sample ofcrystals is measured to be 64 :775:1 counts/keV/kg/day in the energy range of5–20 keV : Quantitative estimation ofresidual radioactive isotope in CsI ðTcÞ was made using the GEANT4 Monte Carlo simulation. Analysis results show that CsIðTcÞ crystal could be a good candidate for direct detection of WIMPs when the contamination level ofcesium radioisotopes is reduced to under a f mBq/kg. r 2003 Elsevier Science B.V. All rights reserved.

Measurements of cosmic-ray proton and helium spectra from the BESS-Polar long-duration balloon flights over Antarctica

The BESS-Polar Collaboration measured the energy spectra of cosmic-ray protons and helium during two long-duration balloon flights over Antarctica in December 2004 and December 2007, at substantially different levels of solar modulation. Proton and helium spectra probe the origin and propagation history of cosmic rays in the galaxy, and are essential to calculations of the expected spectra of cosmic-ray antiprotons, positrons, and electrons from interactions of primary cosmic-ray nuclei with the interstellar gas, and to calculations of atmospheric muons and neutrinos. We report absolute spectra at the top of the atmosphere for cosmic-ray protons in the kinetic energy range 0.2-160 GeV and helium nuclei 0.15-80 GeV/nucleon. The corresponding magnetic rigidity ranges are 0.6-160 GV for protons and 1.1-160 GV for helium. These spectra are compared to measurements from previous BESS flights and from ATIC-2, PAMELA, and AMS-02. We also report the ratio of the proton and helium fluxes from 1.1 GV to 160 GV and compare to ratios from PAMELA and AMS-02.

MEASUREMENTS OF THE RELATIVE ABUNDANCES OF HIGH-ENERGY COSMIC-RAY NUCLEI IN THE TeV/NUCLEON REGION

We present measurements of the relative abundances of cosmic-ray nuclei in the energy range of 500-3980 GeV/nucleon from the second flight of the Cosmic Ray Energetics And Mass balloon-borne experiment. Particle energy was determined using a sampling tungsten/scintillating-fiber calorimeter, while particle charge was identified precisely with a dual-layer silicon charge detector installed for this flight. The resulting element ratios C/O, N/O, Ne/O, Mg/O, Si/O, and Fe/O at the top of atmosphere are 0.919 ? 0.123stat ? 0.030syst, 0.076 ? 0.019stat ? 0.013syst, 0.115 ? 0.031stat ? 0.004syst, 0.153 ? 0.039stat ? 0.005syst, 0.180 ? 0.045stat ? 0.006syst, and 0.139?? 0.043stat ? 0.005syst, respectively, which agree with measurements at lower energies. The source abundance of N/O is found to be 0.054 ? 0.013stat ? 0.009syst+0.010esc ?0.017. The cosmic-ray source abundances are compared to local Galactic (LG) abundances as a function of first ionization potential and as a function of condensation temperature. At high energies the trend that the cosmic-ray source abundances at large ionization potential or low condensation temperature are suppressed compared to their LG abundances continues. Therefore, the injection mechanism must be the same at TeV/nucleon energies as at the lower energies measured by HEAO-3, CRN, and TRACER. Furthermore, the cosmic-ray source abundances are compared to a mixture of 80% solar system abundances and 20% massive stellar outflow (MSO) as a function of atomic mass. The good agreement with TIGER measurements at lower energies confirms the existence of a substantial fraction of MSO material required in the ~TeV per nucleon region.

Development of Low Background CsI(Tl) Crystals and Search for WIMP

The response of CsI(Tl) crystals to nuclear recoils and gammas down to 3 keV has been studied, and it has been demonstrated that the pulse shape discrimination capability for CsI(Tl) is better than that for NaI(Tl). Extensive study of internal background of CsI powder was performed and a lot of efforts have been made to reduce the internal background sources such as 137Cs and 87Rb. The 137Cs concentration of 2 mBq/kg was achieved successfully in the CsI powder using ultra-pure water for whole extraction process. The Rb concentration was successfully reduced to be less than 1 ppb by the recrystallization method. The CsI(Tl) crystal used for the experiment was grown with the ultra-low background CsI powder and a dimension of the CsI(Tl) crystal is 8times8times30 cm3 which corresponds to weight of 8.7 kg. The number of photoelectrons is measured to be about 4-6/keV with a RbCs photocathode PMT. The background of CsI(Tl) crystals were measured in the shielding of 30 cm thick mineral oil, 15 cm thick lead, 5 cm thick polyethylene and 10 cm copper at the 700 m minimum depth of underground laboratory at YangYang Pumped Storage Power Plant in Korea. The WIMP search with low background CsI(Tl) crystals has been carried out by the Korean Invisible Mass Search (KIMS) Collaboration and the lower limit on WIMP-nucleon cross section are reported.